This invention relates to a process for the production of a liquid crystal display device having a color filter. More specifically, this invention relates to such a process which enables the color filter of such a display to be formed after assembly of the display is complete or almost complete. This invention also relates to a second process which enables the formation of an opaque grid in association with an existing color filter.
In recent years, much research has been devoted to liquid crystal display devices, which are now the accepted form of both monochrome and color screens in portable computers and may become the accepted form of screens in desk-top computers because, for a given screen area, they have a much smaller footprint and power requirement than cathode ray tubes.
Although different types of color liquid crystal display devices exist, in general such devices comprises two spaced panels (which may hereinafter be called "support members") which define a sealed cavity filled with a liquid crystal material. A plurality of electrodes are formed on one of the panels and one or more electrodes on the other panel; in the so-called "active matrix" type of display, a single electrode is typically formed on one panel, while the other panel bears a grid of electrodes each of which defines a single pixel of the display. One of the panels carries a color filter containing filter elements of three different colors, typically red, green and blue. Each filter element is aligned with one of the pixels defined by the electrodes and a set of adjacent red, green and blue filter elements is grouped into a triad or other consistent pattern, such as repeating stripes.
Depending upon the image to be displayed, one of more of the electrodes are energized and, depending upon the nature of the liquid crystal material and the level of energization, the transmissivity of the liquid crystal material between the electrodes is altered to allow all light, no light or part of the light to be transmitted through the associated color filter. The user perceives the image as a blending of colors formed by the transmission of light through the various filter elements.
The display may be backlit by providing a light source on the opposite side of the display device from the user. Alternatively, the display device may include a reflective layer on its rear surface and rely upon reflection from a light source located on the same side of the display as the viewer.
The requirements for forming the filter elements on a liquid crystal display device are stringent. A typical 10 inch (254 mm) display (measured diagonally) having a VGA resolution of 640 by 480 pixels requires approximately 80 pixels per inch (about 3.1 per mm) in both directions. A slight misalignment (say 0.2 pixel width, or about 0.06 mm) between the filter elements and the corresponding electrodes will result in changes in color that are readily apparent to the user.
Numerous techniques have been employed to form such color filters, including the use of photoresists, deposition of organic pigments by vacuum evaporation followed by conventional photolithographic lift-off techniques, the use of dyed and patterned stretched film material to create an internal color polarizing filter, and the use of conventional silver halide photosensitive layers. For example, U.S. Pat. No. 4,743,098 describes a liquid crystal display device in which the color filter is formed from polychromatic glass. Through a series of masking, ultraviolet exposure and heat treating steps, red, green and blue areas are formed in the polychromatic glass, which forms one of the support members of the display device.
U.S. Pat. No. 5,229,232 describes the use of a thermal printing system in which various patterned masks are used in conjunction with an electronic flash and other related hardware to thermally transfer dyes to a receiver so as to create a color filter array with an incorporated black matrix for use in a color liquid crystal display device.
U.S. Pat. No. 5,053,298 describes a method of manufacturing a color filter comprising: coating a transparent planarizing layer on a substrate having electrodes formed thereon, the planarizing layer serving to flatten the surface of the substrate; forming on the planarizing layer a first relief pattern overlapping a light receiving region corresponding to a first color in the filter to be formed; coloring this first relief pattern with the first color; forming a medium layer to prevent coloring, this medium layer covering the whole surface of the structure resulting from the coloring step; forming openings to expose the planarizing layer; forming a new relief pattern within the openings, this new relief pattern overlapping another light receiving region corresponding to a second color in the filter to be formed; coloring this relief pattern with a color different from the first color; removing the medium layer; forming colored relief patterns with different colors by successively repeating the steps of forming a medium layer, forming openings, forming a new relief pattern overlapping a light receiving region, coloring the newly formed relief pattern and removing the medium layer; and coating a protective surface over the whole surface of the resultant structure after foraging the colored pattern for the last desired color.
U.S. Pat. No. 5,093,738 describes a method of forming a color filter substrate having pixels of at least two different colors by use of negative photoresist material of the last color deposited. The negative photoresist is coated over the pixels and then exposed to a light source at a wavelength blocked by the formed pixels so that any of the negative photoresist material directly on the transparent substrate is activated by the light from the light source and the remaining photoresist material is thereafter removed.
As will be apparent from the foregoing description, many of the prior art techniques for forming color filters in liquid crystal display devices are complex, multistep procedures susceptible to error, and since these techniques are practiced on devices which have already undergone the complex and expensive procedure for forming the electrodes, any failures in the filter-forming process result in scrapping the expensive electrode assembly. Furthermore, because the prior art techniques require direct access to the filter-forming layer (and in many cases require such access by liquid developing solutions), the filter-forming process must be carried out on an incomplete display device, thus leaving the filter susceptible to damage and/or contamination during the steps necessary to complete the manufacture of the display device.
The present invention provides a process for forming a color filter in a liquid crystal display device which requires only a small number of steps and which can be carried out with the display device completely assembled, thus avoiding any risk that the filter will be damaged or contaminated during later processing or assembly. Preferred forms of the present invention also ensure perfect alignment between the filter elements and the pixels. This invention also provides a process for the formation of an opaque grid in association with an existing color filter.